Shaped article of ceramic material



Jan. 3, 1956 H. SCHARF 2,729,422

SHAPED ARTICLE OF CERAMIC MATERIAL Filed March 26, 1952 INVENTOR HERBERTScnmzr ATTORNEYS SHAPED ARTICLE OF CERAMIC MATERIAL Herbert Scharf,Augsburg, Germany, assignor to Maschinenfabrik Augsburg-Numberg A. G.,Augsburg, Germany, a corporation of Germany Application March 26, 1952,Serial No. 278,559 Claims priority, application Germany April 6, 1951 4Claims. 01. 253-77 This invention relates to certain improvements in theshaping of engine parts or machine elements of ceramic material, moreparticularly, in the form of blades for gas turbines.

In engineering practice, ceramic materials are used primarily where inaddition to mechanical stresses the parts are exposed to very hightemperatures exceeding the upper temperature limit admissible formetallic materials. The mechanical strength of shaped articles ofceramic materials is not substantially changed by heating them from roomtemperature to very high temperature. However, in view of the relativelylow mechanical strength of ceramic materials, it is necessary to payspecial attention to the factors determining the rated fatigue limit ofthe respective part. This is especially true in case of highly stressedengine parts, such as, turbine blades or vanes. The main difiicultyconsists in the fact that the slightest peak strains which have beenneglected in the design will cause break-down of the ceramic element ifsuch peak load exceeds the strength of the material.

More particularly, it has been found that the equilibrium of thestructural or internal strain which originates from the firing and is acharacteristic feature of ceramic materials is disturbed by themachining of individual surfaces of the ceramic part, for instance bygrinding, whereby the rated fatigue limit is substantially reduced. Thecritical points of a partly ground workpiece are at the transitionsbetween the ground surface and the non-injured or unwrought naturalsurface or skin of the shard produced by the firing.

It is an object of the present invention to provide a design whichoffers optimum conditions as to the mechanical strength of engine partsor other shaped articles of the type referred to.

With this and further objects in view, according to the presentinvention the place of transition between the machined supporting orseating surface or surfaces and the unwrought surface or surfaces of theengine part, more particularly, of the root of a turbine blade, isshaped in the form of a hollow or flute. According to a further featureof the invention, such flute is arranged to have its center point, orits center axis, respectively, on a plane which is flush, or nearlyflush, with the ground surface. It has been found that the deleteriousinfluence of ground surfaces on the mechanical strength can becompensated particularly eifectively in this way. Generally speaking,the ground surface should form a right angle with the returningnon-ground surface. The machining or tooling allowance which isindispensable with ceramic materials for rendering it possible to adheresafely to the dimensions indicated in the drawing in spite of the largetolerances caused by the manufacturing process, and by the largeshrinking diflerences inherent to this material, should be dimensionedso that after grinding to the specified measure the centers of theflutes are not located substantially above the plane of the groundsurfaces. The important thing is to arrange the surfaces in such a waythat the compressive forces act on the machined surfaces 2,729,422iiatented Jan. 3, 1956 in a normal direction while non-stressed parts ofthe shard adjacent to the surface acted upon by the force are avoided.

It will be understood that the invention is not restricted to itsapplication in ceramic turbine blades but it may be used for otherceramic engine parts or machine elements as well.

Other and further objects, features and advantages of the invention willbe pointed out hereinafter and appear in the appended claims formingpart of the application:

One embodiment of the invention is illustrated by way of example andpurely schematically in the accompanying drawing, showing a fragmentarysectional view of a conventional turbine wheel and a ceramic turbineblade seated in a dovetail groove of the turbine wheel.

Referring to the drawing, it will be seen that the turbine blade,consisting of a blade portion 1 and a root portion 2, is inserted in adovetail groove 3 of a turbine rotor 4 and supported by means of itsmain seating surfaces or load carrying faces 5 on the supportingsurfaces of the rotor 4.

Adjacent to the ground seating surfaces 5 the unwrought surface of theroot of the blade is provided with hollows or flutes 8, the centers X ofthe circles defining the curvature of the flutes 8 being located in thesame plane as the respective ground seating surfaces 5. It is notabsolutely necessary for the flute to be cylindrical or for its centeraxis to be disposed accurately in the plane of the seating surface. Theimportant thing is that the compressive forces P acting upon the seatingsurface 5 are introduced into these surfaces in a normal, i. e.,perpendicular direction.

It will be noted from the dotted line 5 that the seating surfaces 5 areprojected or extended outwardly from the conventional wedge shape bladeroot construction, and that, as noted above, a tangent T to the flutesor hollows 8 at the point of intersection thereof with the seatingsurfaces 5 forms a right angle with the seating surfaces.

While the invention has been described in detail with respect to a nowpreferred example and embodiment of the invention it will be understoodby those skilled in the art after understanding the invention thatvarious changes and modifications may be made without departing from thespirit and scope of the invention and it is intended, therefore, tocover all such changes and modifications in the appended claims.

What is claimed is:

1. A ceramic turbine blade having a working portion and a root portionextending therefrom for affixing said blade in position in use in agroove in the rim of a turbine, said root portion having a generallywedge shape dovetail configuration outwardly flared from said workingportion and including load carrying surfaces outward ly projecting fromsaid dovetail configuration and positioned to receive perpendicularlythereto substantially the entire forces acting on said root portion inuse, said outwardly projecting surfaces being machine finished andbounded on opposite sides by concave arcuate walls which are not machinefinished and the radii of curvature of which do not substantially exceedthe extent of projection of said surfaces, and a tangent to said concavewalls at the points of intersection with said load carrying surfacesforming substantially a right angle with said surfaces.

2. In a turbine having ceramic blades aflixed around the rim of aturbine rotor, the combination which comprises seating surfaces in saidrotor rim, corresponding dovetail seating surfaces on said blade forload bearing engagement with said rotor'seating surfaces, said bladeseating surfaces being outwardly and downwardly projecting from saidblade and having accurately machined surfaces for fitting engagementwith said rotor surfaces,

said outwardly projecting blade seating surfaces being bounded on.opposite sides by v concave curved wall portions the centers ofcurvature of which are substantially in the planes of said projectingseating surfaces, and said curved walls being unrnachined and 1 meetingsaid seating;

lie/substantially nofurther from thecenter line of saidceramic partthan-saidmachine finished portions, and said concaveiarcuate wallsmeeting said machine finished portions at substantially a right angle.

4. A. ceramic machine part subject to substantial load stress inusecomprising a machine finished surface portion of substantial area fortransmitting said load stress,

another. surface portion which is-notmachine finishedand extendsbeyond'said machine finished area, and concave arcuate walls joiningsaid machine finished portion with said unmachined'portion, the centersof curvature ofsaid concavewallslying substantially no further from thecenter line of said ceramic part than said machine finished portion andsaid concave arcuate walls being unmaehined and meeting said machinefinished portions at substantially a right angle.

References-Cited in the file of this patent UNITED STATES. PATENTS1,491,079 Champion Apr. 22, 1924 2,118,795 Littleton May 24, 19382,183,699 Schweiker Dec. 19, 1939 2,317,338,- Rydmark Apr. 20, 19432,363,448 Siesel Nov. 21, 1944 FOREIGN PATENTS 664,986 Great BritainJan. 16, 1952 821,735 Germany Nov.,19, 1951 892,785 France May 19, 1944OTHER REFERENCES Ser; No. 285,333; Schutte' (A. P; C.), published May25; 1943.

Ser. No. 385,334, Schutte; (A. 1 0), published May 25', 1943;

